Superconductivity is a phenomenon of zero or near-zero electrical resistance and expulsion of magnetic fields occurring in certain materials when cooled below a characteristic critical temperature. Josephson junctions (JJ) are non-linear, loss-less circuit elements which utilize the Josephson effect and are important to superconducting technology. Superconducting wires and Josephson junctions, along with other electrical elements such as capacitors and inductors, can be combined to construct superconducting circuits. The wide spread use of the JJ follows a long history of development including the IBM Josephson digital computer program of the 1970s, which pioneered the tunneling JJ technology. This program was based primarily on Pb-alloy tunnel junctions whose applicability is limited due to the critical current spread and the instability of the Pb. Since then, Niobium/Aluminum-Oxide/Niobium (Nb/AlOx/Nb) Josephson junctions and Aluminum/Aluminum-Oxide/Aluminum (Al/AlOx/Al) Josephson junctions have become almost ubiquitous for superconducting devices such as superconducting qubits, logic for classical information processing, sensors such as Superconducting Quantum Interference Device (SQUID) magnetometers, and circuits used to set voltage standards. Nb/AlOx/Nb JJs proved to be more reliable and stable and have become the materials of choice in many traditional superconducting elements and devices, while Al/AlOx/Al JJs are typically preferred for quantum computing and quantum device applications at millikelvin-operating temperatures as in a dilution refrigerator.
Nb/AlOx/Nb and Al/AlOx/Al JJs are made out of heterogeneous components, i.e. they are formed from some combination of superconducting metals, insulators (such as between the two superconductors to form a tunnel barrier), oxides that form naturally on the surface of the superconductors, or the substrate in which the elements are fabricated. Similarly most superconducting elements are made out of heterogeneous components.
Josephson junctions are essential elements for many superconducting applications. Two types of Josephson junctions include the superconducting tunnel junction and the weak-link junction. Each of these Josephson junctions varies in the way the two superconducting components are connected. While the two superconducting components of a tunneling Josephson junction are divided by a tunneling barrier, in the weak-link JJ the two superconducting components are connected by a superconducting or metallic (normal metal) link.
A prior art tunneling Josephson junction 100 is illustrated in FIG. 1. As illustrated, the Josephson junction 100 includes a first superconducting component 102, a second superconducting component 104, and a tunneling barrier 106 positioned between the first and second superconducting components 102, 104. The first and second superconducting components 102, 104 are formed on a substrate 108. Given the heterogeneous components of the Josephson junction 100, a number of interfaces result. These interfaces are emphasized in FIG. 1 by using a “cross-hatched” notation. A substrate-superconductor interface 110 is provided where the superconducting components 102, 104 contact the substrate 108; a barrier interface 112 is provided where the superconducting components 102, 104 contact the barrier 106; a substrate-vacuum interface 116 is provided where the substrate 108 contacts the vacuum in which the JJ resides; and a superconductor-vacuum interface 114 is provided where the components of the Josephson junction are exposed to the vacuum within which the Josephson junction resides. Loss or noise which limits device performance can occur at each of these interfaces or within the non-superconducting materials such as the substrate 108 or the tunnel barrier 106. For example, at each of the interfaces identified, charge traps, dangling bonds, impurities, or other defects resulting from an imperfect match between the two materials can result in, for example, two-level fluctuators that can absorb electromagnetic energy (loss) or create fluctuating electromagnetic fields or unwanted quantum interactions (noise). Within the substrate or barrier material, which could be crystalline or amorphous, impurities and other defects can also cause loss and noise. At the superconductor-vacuum interface 114, a surface oxide on the metal surface often forms which can cause similar loss and noise problems.
A prior art weak link Josephson junction is illustrated in FIG. 2. As illustrated the weak link Josephson junction 200 includes a first superconducting component 202 a second superconducting component 204, and a weak link 206 positioned between the first and second superconducting components 202, 204. The first and second superconducting components 202, 204 are formed on a substrate 208 and are spaced apart by a region. The weak link 206 is positioned within this region and opposite ends of the weak link 206 respectively contact the first and second superconducting components 202, 204. The weak link 206 is formed from a normal metal region or a superconducting region where the superconductivity is weakened either by geometric constriction or by a change in the superconducting order parameter. Given the heterogeneous components of the weak link Josephson junction 200, a number of interfaces result. These interfaces are emphasized in FIG. 2 by using a “cross-hatched” notation. A substrate-superconductor interface 210 is provided where the superconducting components 202, 204 contact the substrate 208; a link interface 212 is provided where the superconducting components 202, 204 contact the link 206; a substrate-vacuum interface 216 is provided where the substrate 208 contacts the vacuum in which the JJ resides; and a superconductor-vacuum interface 214 is provided where the components of the Josephson junction 200 are exposed to the vacuum within which the Josephson junction 200 resides. Loss or noise which limits device performance can occur at each of these interfaces or within the non-superconducting materials such as the substrate 208. For example, at the substrate-superconductor interface 210 or link interface 212, charge traps, dangling bonds, impurities, or other defects resulting from an imperfect match between the two materials can result in, for example, two-level fluctuators that can absorb electromagnetic energy (loss) or create fluctuating electromagnetic fields or unwanted quantum interactions (noise). Within the substrate 208, which could be crystalline or amorphous, impurities and other defects can also cause loss and noise. At the superconductor-vacuum interface 214, a surface oxide on the metal surface often forms which can cause similar loss and noise problems.
Traditionally, the superconducting tunneling junction has been widely used due to its easier fabrication with AlOx barrier and its well-defined nonlinear current-phase relation. Weak-link junctions traditionally provide an alternative in applications requiring high Josephson critical current and/or small size junction areas. In some applications, such as for example, quantum computing or other low-power circuits, interfaces between the materials used to form the heterogeneous superconducting elements pose problems. For example, problems at the interfaces provided by these devices lead to electromagnetic loss or electromagnetic or quantum noise and subsequent poor performance of the circuits which incorporate these devices and elements. Specifically, for example in the case of heterogeneous superconducting qubits, electromagnetic loss occurs at the substrate-vacuum interface resulting in qubit coherence problems, which relates to memory and gate errors. This error limits applications in which superconducting qubits may be utilized.
Another problem with heterogeneous superconducting devices is that formation of these heterogeneous superconducting devices requires the fabrication of different materials (e.g. metals, insulators, crystals, and substrates) making fabrication more difficult.
Another problem with heterogeneous superconducting devices is that sometimes the material used limits the device which can be constructed. For example, if metal wires are to be formed, certain fabrication techniques for metals limit the minimum size for the metal wires.
The invention builds from experimental progress in four different areas. First, the list of superconducting materials has expanded to include doped covalent semiconductors, particularly silicon (Si) and germanium (Ge). These techniques provide for heavily doping the semiconductor material with acceptors, for example Boron (B), in bulk or thin-film samples. By doping a semiconductor or an insulator above the metal-insulator transition density, the host material can turn into a superconductor below a critical temperature and magnetic field. Superconductivity has been observed in many materials including hole-doped, group-IV materials such as diamond, silicon and germanium. Various methods, such as high-pressure high-temperature treatments and growth using chemical vapor deposition (CVD) for C:B, gas-immersion laser doping (GILD) for Si:B, and ion implantation and annealing for Si:Ga and Ge:Ga were used to achieve very high hole densities required for superconductivity.
A second area of progress is in precise doping techniques resulting in the ability to form single atom thick sheets of impurities, single atom-wide wires, a single phosphorus atom at precise atomic locations within a crystal, and single atom-depth and/or single-atom wide wires on different vertical planes of a crystal. For example, scanning tunneling microscope (STM) lithography together with atomic layer doping has been used to achieve very high, atomically precise doping of the donor phosphorous (P) in Si and Ge.
A third area of development provides for the formation of spin qubits in silicon. This development provides that a single electron or hole spin is attached to single donor or trapped in a quantum dot in isotopically enriched and chemically purified silicon. These spin qubits have been demonstrated to have very good coherence properties, and therefore are widely considered to be leading candidates for fault-tolerant quantum computing.
Fourth, the development of superconducting circuit technology resulting in a large design space of superconducting qubit approaches and approaches for low-powered superconducting Josephson junction based logic, sensors, and other superconducting devices.